1. Field of the Invention
This invention relates to water-based chemical compositions and is concerned, in particular, with water-based inks and varnishes, such as are used in flexographic and gravure printing and other specialised printing and overprinting processes.
2. Description of Prior Art
There is a general need for compositions for such purposes which are essentially aqueous in nature, i.e. they are water-based instead of being based upon organic solvents. Important factors in this respect include the general expense of organic solvents, environmental pollution problems caused by the use of organic solvents, both during manufacture and in the use of solvent-based systems, and the increasing scope and effect of health and safety legislation. In addition to the important reasons, such as those just described, underlying the need for water-based alternatives to solvent-based compositions, new uses have been developed in recent times for water-based flexographic and gravure inks and overprint varnishes, for printing upon absorbent paper and other absorbent substrates such as board materials used in packaging. One particular need which is now generally recognised is the development of water-dispersible media, for application to impermeable substrates, with the same qualitative results as are attainable with conventional solvent-based systems, especially in relation to adhesion and to chemical and water resistance properties. Moreover, it is a purpose of this invention to provide water-based compositions which have improved properties as compared with known water-based ones, both as regards cost and technical effect. For instance, known water-based inks usually comprise acrylic polymers, ranging in cost from 1,600 to 4,000 per metric tonne, and such inks sometimes lack the desired degree of adhesiveness to certain substrates. Printing applications with water-based acrylic polymer-containing inks which produce less than ideal results include the use of flexible substrates, such as plastics sheets used in the manufacture of multicolour printed plastics bags and similar products. It has now been discovered that improved water-based chemical compositions having a very desirable and useful range of properties can be made by using dispersions of rosin and/or rosin derivatives as extenders for organic polymer-based aqueous compositions. The invention thus provides compositions which are eminently useful as inks for such special uses as in flexographic and gravure processes, which match or even improve upon the properties of known solvent-based compositions, which substantially reduce the disadvantageous effects of organic solvents and which significantly reduce costs, by replacing large proportions of the relatively expensive organic polymers used in such compositions, but without causing any lowering of qualitative properties and standards.